Radiotherapy is used to treat cancers and other ailments by irradiating tissue with ionizing radiation. Radiotherapy systems generate a beam of radiation (e.g. electrons, protons, ions, and the like) and direct the beam towards a target site, such as a tumour. To concentrate the radiation at the target site and to minimize irradiation of healthy surrounding tissue, radiotherapy systems often also include a beam-shaping device such as a multi-leaf collimator (MLC). A MLC includes rows of elongate leaves that are arranged side-to-side and constructed of a radiation-shielding material such as tungsten. Each leaf can be independently moved into and out of the path of the radiation to block a portion of the beam. By arranging the collimator leaves, the MLC can be used to shape the radiation beam in order to focus the dose on the target tissues.
Given the importance of accurately controlling the beam shape, techniques have been developed to calibrate the positions of collimator leaves. For example, some radiation-based calibration techniques utilize x-ray film or point dosimeters to confirm that the leaves form the desired radiation beam shape. However, such techniques can be time-consuming and often provide a poor indication of the actual beam geometry. Other calibration techniques involve using a laser beam and optical detector to determine when the MLC leaves have reached a defined calibration position. However, this technique may not provide an accurate indication of the leaf positions for all leaf shape configurations. Still further calibration techniques involve imaging optical markers on the leaves with a camera and using the detected positions of the optical markers to determine the positions of the leaves. However, the lens of the camera can distort the images of the markers, meaning additional calibration steps may be necessary to provide accurate determination of the leaf positions. The extent of this distortion is different for each lens and can change every time adjustments are made to the camera; thus, a distortion correction technique developed for one camera may not be applicable to other cameras, or to the camera in question after servicing. In addition, because the optical markers are manually placed on the collimator leaves, the distance between the marker and the leaf tip (a distance known as the “minor offset”) is different for each leaf. Existing MLCs cannot simply measure the minor offset with the camera because the leaves are not visible to the camera. For these reasons, existing collimator systems may require computationally-intensive and time-consuming calibration steps to ensure that collimator leaves are correctly positioned during radiotherapy.
Thus, there remains a need for improved techniques for accurately monitoring collimator leaf positions by minimizing the distortion of leaf images caused by the camera lens and by accurately measuring and accounting for the minor offsets of the leaves in a more timely fashion. The present disclosure provides systems and methods for generating undistorted images of collimator leaves and accurate measurements of the minor offsets using a minimal number of measurements, such that the true positions of the leaves can be determined without adding excessive calibration time to the machine setup process. As a result, the leaves can be even more accurately placed during radiotherapy so that the desired beam geometry can be achieved and the time required to perform the calibration may be reduced.